The four-carbon compound 1-butanol has considerable value and utility as both a liquid fuel and as a commodity chemical. In particular, there is now increased interest in using biobutanol as a transport fuel. Various methods have been used to produce 1-butanol, including catalyzed reactions from the petrochemical feedstock propylene, catalyzed ethanol dimerization and ABE fermentation, all of which have significant disadvantages. New methods for butanol, in particular biobutanol production are urgently needed.
1-butanol is most commonly produced from the petrochemical feedstock propylene. In the presence of hydrogen and carbon monoxide, propylene undergoes hydroformylation using a cobalt or rhodium catalyst. The process requires temperatures of 100° C. to 200° C. and pressures up to 300 atm, and produces a mixture of approximately 88% 1-butanol and 12% iso-butanol. The reaction is illustrated below.

Ethanol, a two-carbon alcohol, can be dimerized using Geurbert chemistry to produce 1-butanol. This allows use of bio-ethanol, so that the 1-butanol produced is also bio-based. Geurbert chemistry has been known since the end of the 19th century, and patents employing that chemistry for the production of 1-butanol from ethanol date to the 1930s (U.S. Pat. No. 1,992,480). The process employs catalysts that perform a series of reactions (oxidation, aldol condensation, dehydration, and reduction) to give the higher alcohol plus a molecule of water. The reaction is illustrated below.

Thus, production of 1-butanol from propylene or ethanol via catalytic reactions requires expensive catalysts and/or harsh reaction conditions.
1-butanol can also be produced via fermentation using Clostridium acetylbutylicum. This fermentation process, called the Acetone-Butanol-Ethanol (ABE) process was patented in the early 20th century by Fernbach and Weizmann, and their processes were patented in 1912 and 1915 respectively; the Weizmann process (British Patent 4845, 6 Mar. 1919) eventually dominated the industrial production of acetone and butanol. The fermentation yields the three solvents, Acetone: 1-Butanol:Ethanol, in an approximate ratio of 3:6:1. The tools of molecular biology have been applied, and a hyper-producing strain Clostridium beijerincki with yields of total solvents up to 165 g/L of 1-butanol has been published. (N. Qureshi, H. P. Blaschek, J. Ind. Microbiol. Biotechnol., 2001, 27, 287-291).
The metabolism of the Clostridia species used for the ABE fermentation is complex, passing through an acidogenic phase in which acetic and butyric acids are generated and excreted from the cell, followed by a solventogenic phase in which the acetic and butyric acids are taken back up by the cell and reduced to give ethanol, acetone, and butanol. Pathways have been engineered which allow the host organism to avoid this complex behavior, and produce 1-butanol from acetyl-CoA using the sequence of enzymes acetyl-CoA acetyltransferase (AtoB), 3-hydroxybutyryl-CoA dehydrogenase (Hdb), 3-hydroxybutyryl-CoA dehydratase (Crt), trans-enoyl-CoA reductase (Ter), and aldehyde/alcohol dehydrogenase (AdhE2) (Shota Atsumi et al., Metabolic Engineering 10 (2008) 305-311).
Regardless of the metabolism of the cell or the pathways present, balanced stoichiometry requires that the carbon efficiency is two-thirds, that is, of the 6 carbon atom present in the starting glucose (C6H12O6) only 4 of then are present in the 1-butanol produced (C4H10O); the remaining 2 carbon atoms being lost as 2 molecules of carbon dioxide. As shown in the balanced equation below, this is the theoretical maximum carbon efficiency possible for the fermentation of glucose to 1-butanol.C6H12O6→C4H10O+2CO2+H2O  Equation I
Biofuels now comprise approximately 10% of the total 130 billion gallons/year US automobile fuel market. By increasing the theoretical carbon efficiency of 1-butanol production from 66.6% to 100%, that is, if all the carbon in the starting glucose could be present in the 1-butanol product, this would increase raw material production yields by 50%. Advanced biofuels such as 1-butanol continue to penetrate the automobile fuel market, targeted at 36 billion gallons by 2022. Increased carbon efficiency could produce 36 billion gallons from the same amount of biomass and starch required to produce 24 billion gallons using today's process technology, making biofuels considerably more competitive.
Thus it is highly desirable to increase the carbon efficiency of the fermentation of glucose to 1-butanol.
Another issue illustrated by Equation I is the production of CO2 which is lost carbon that is not transformed to the desired 1-butanol product. The loss of CO2 from both biological and non-biological processes and the desirability of recovering the CO2 has been recognized for many years (P. G. Russell et al., J. Electrochem. Soc. 1977, 124(9), 1329-1338). Reduction of CO2 to methanol has been published (Robyn Obert et al., J. Am. Chem. Soc. 1999, 121, 12192-12193; Song-wei Xu et al., Ind. Eng. Chem. Res. 2006, 45, 4567-4573; Xiaoli Wang et al., ACS Catal. 2014, 4, 962-972; Torsten Reda et al., PNAS 2008 105(31), 10654-10658) and patented (U.S. Pat. No. 6,440,711 B1, Aug. 27, 2002) using electrochemical (Neil S. Spinner et al., Catal. Sci. Technol., 2012, 2, 19-28), photochemical (Michele Aresta et al., Beilstein J. Org. Chem. 2014, 10, 2556-2565) and standard chemical methods.
Utilization of reducing equivalents produced electrochemically has been proposed for a form of artificial photosynthesis, in which the electrochemically supplied reducing equivalents replace those that would be normally provided by photosystem I. (In Recent Advances in Post-Combustion CO2 Capture Chemistry; Attalla, M.; ACS Symposium Series; American Chemical Society: Washington, D.C., 2012). However, this proposal simply proceeded to generate starch, plus taking some of the pyruvate to ethanol and CO2 in order to balance ATP requirements.
Improving carbon efficiency by the capture of CO2 has been explored for from industrial waste gases (Michele Aresta, Angela Dibenedetto, Antonella Angelini, Chem. Rev., 2014, 114 (3), 1709-1742; Michele Aresta, Angela Dibenedetto, Dalton Trans., 2007, 2975-2992), through the reduction of CO2 to formate, with the accompanying issues about gas transfer in liquids, and the need to form carbonic acid and bicarbonate from CO2 prior to the reduction to formate.
The reduction of CO2 to formate is energetically unfavorable (Colin Finn, Sorcha Schnittger, Lesley J. Yellowlees, Jason B. Love, Chem. Commun., 2012, 48, 1392-1399; F. Suhan Baskaya, Xueyan Zhao, Michael C. Flickinger, Ping Wang, Appl Biochem Biotechnol (2010) 162:391-398), and when coupled with the possible need to utilize carbonic anhydrase to first catalyze the conversion of CO2 to carbonic acid to avoid the problem of gas transfer in liquids (Paul K. Addo, Robert L. Arechederra, Abdul Waheed, James D. Shoemaker, William S. Sly, Shelley D. Minteer, Electrochemical and Solid-State Letters, 14 (4) E9-E13 (2011)), the capture of CO2 as a method for improving carbon efficiency appears unattractive in any system.
Thus, there is a need for an improved system and method for 1-butanol production that addresses all of the above issues, with decreased production cost (by, e.g., avoiding expensive catalysts), increased carbon efficiency and avoidance of CO2 production.